The present invention relates to a liquid transfer pump mainly used for transferring a mobile phase or liquid to a column in a liquid chromatography, wherein, in particularly, axial centers of a plunger and a plunger seal always coincide with each other by a free movement of the plunger in the radial directions, so that the sealing life of the plunger seal is prolonged without damaging the plunger seal, and also, the plunger seal and a plunger end can be washed by water in case eluent containing salt as a mobile phase is used.
In the liquid chromatography, a mobile phase solvent to be transferred to a column is transferred by a pump. As a pump, a reciprocating type pump, i.e. piston type fixed flow amount pump or metering pump, which can provide a high pressure output with a precise flow amount, is most likely used.
In a piston type fixed flow amount pump or metering pump 40, as shown in an explanatory drawing in FIG. 5, for example, rotation of a driving motor 41 is transferred to an eccentric cam 43 to push a plunger 45 held by a plunger holder 42 through a follower 46, so that the mobile phase solvent is ejected with high pressure to a column side. Further, when the eccentric cam 43 rotates for 180 degrees, a return spring 44 pushes the plunger 45 backwardly, so that the mobile phase solvent is inhaled from a reservoir for the mobile phase into a pump chamber 40a. An inlet check valve 47 and an outlet check valve 48 are installed to prevent counter flows of the intake and output of the mobile phase solvent due to the reciprocal movements of the plunger 45.
In the reciprocal movements of the plunger, when the inner pressure in the pump chamber 40a becomes lower than the atmospheric pressure, the outlet check valve 48 is closed and the inlet check valve 47 is opened, so that the mobile phase solvent flows into the pump chamber 40a. When the inner pressure in the pump chamber 40a becomes higher than the atmospheric pressure, the outlet check valve 48 is opened and the inlet check valve 47 is closed, so that the mobile phase solvent is ejected from the pump chamber 40a to the column side.
In another conventional liquid transfer pump used in a liquid chromatography, as shown in FIG. 4, a plunger rod 52a is fixed, i.e. adhered, to a plunger holder 52, and the plunger holder 52 is screwed or fixed to a cross head 55. Also, the plunger rod 52a passes through a plunger seal 53 attached to a side of a pump head 51 and is held thereat, so that the mobile phase solvent does not leak therethrough.
On the other hand, whether or not axial centers of the plunger rod 52a and the cross head 55 coincide with each other depends on accuracy in the mechanical processing for the plunger rod 52a, plunger holder 52 and cross head 55. Thus, if the accuracy of the mechanical processing is not good, an improper or excessive force is applied from the plunger rod 52a to the plunger seal 53, which affects a bad influence to the life of the plunger seal 53.
Namely, the life of the plunger seal 53 depends on the accuracy of the mechanical processing of the plunger rod 52a and the plunger holder 52, and is not stable.
The present invention has been made in view of the above problems, and an object of the invention is to provide a liquid transfer pump, wherein axial centers of a plunger and a plunger seal always coincide with each other by a free movement of the plunger in the radial directions, so that an improper or excessive force is not applied to the plunger seal. Namely, in the liquid transfer pump of the invention, it is possible to prolong the life of the plunger seal without requiring the accuracy of the mechanical processing, and the possibility of leakage through the seal is lowered.